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Thursday, February 09, 2006

Scientists Force Evolution in the Lab

Scientists have forced a little evolution in the laboratory, controlling whether a caterpillar becomes green or black.

The color of the critter was made to vary with temperature during their development. The experiment reveals the basic hormonal mechanism underlying the evolution of such dual traits, the researchers report in the Feb. 3 issue of the journal Science.

The study was done on Manduca sexta, a caterpillar commonly called the tobacco hornworm. Its larvae are normally green. A related species, Manduca quinquemaculata, becomes black or green depending on temperature. The idea was to use similar temperature shocks to evolve a similar change in M. sexta.

Differing color traits induced by environmental factors are called polyphenisms.

Similar differences show up in genetically identical ants, which can develop into queens, soldiers, or workers based on the hormones they're exposed to early in development. Similar hormonal differences can affect the specific color of a butterfly or bird.

Scientists have not understood evolution's exact role in the differences.

"There had been theoretical models to explain the evolutionary mechanism -- how selective pressures can maintain polyphenisms in a population, and why they don't converge gradually into one form or another," said Duke University graduate student Yuichiro Suzuki. "But nobody had ever started with a species that didn't have a polyphenism and generated a brand-new polyphenism."

Suzuki and biology professor Frederik Nijhout worked with black mutants of the normally green M. sexta. The mutants have a lower level of a key hormone.

The scientists subjected the black mutants to temperatures above 83 degrees Fahrenheit, and over a few generations two types developed. One group turned green and the other didn't.

Importantly, the two groups were found to have distinctly different levels of the hormones.

They then found that they could create green spots on black caterpillars by applying drops of the hormones at the right stage of development. And by thwarting the flow of hormones from head to body—they applied a little caterpillar tourniquet—they could prevent the greening.

None of this looks to be going anywhere in the sense of survival of the fittest. The black and green caterpillars will all grow up basically the same.

"The adult moths are identical, and so there is no obvious basis for the kind of selective mating that might genetically isolate two groups and eventually lead to new species," Nijhout told LiveScience. Because the variations are based on temperatures, and thus in the wild would be dependent on seasons, the two types would tend to occur at different times of the year and may never meet in nature, he said.

The next step, the researchers said, is to see if the variations do indeed occur in the wild.

This appears to be an interesting, if somewhat limited experiment. Selection for the green morph of the black mutant using high temperatures could mimic what might happen in the event of global warming, especially given the normal range of Manduca sexta.

But, the author of this Live Science article makes a profoundly dumb statement:

None of this looks to be going anywhere in the sense of survival of the fittest. The black and green caterpillars will all grow up basically the same.

Except that selection can operate on the larval stage just as easily as on the adult stage, especially if the dark mutant is more visible to predators. By analogy with American rat snakes, it seems likely to me that the dark mutants (which are expressed at cooler temperatures) may gain a selective advantage by absorbing more sunlight, thus warming them more quickly in the early morning when their leaf food supply is at its highest nutritional content, whereas the green morph would be more cryptic. In other words, which morph is selected for depends on several interacting environmental factors, including ambient temperature and the presence of avian predators.

Indeed, this experiment is right in line with Mary Jane West-Eberhard's work on evolutionary developmental biology ("evo-devo"). Selection on early developmental stages, such as these larvae, can cause rather dramatic changes in a relatively short time (i.e. "a few generations," as described in the article). Furthermore, the results indicate that polyphenisms (i.e. polymorphisms) can be generated and maintained without necessarily requiring the kinds of genetic mechanisms specified by R. A. Fisher and other population geneticists of the "modern evolutionary synthesis." That is, these experimental results provide evidence for a new paradigm for phenotypic variation, supporting evo-devo and transcending the "modern synthesis."

Now, an "intelligent design theorist" might argue that this doesn't really show anything, as the underlying genetic predisposition for the green color morph was probably already present in the Manduca sexta, and was simply "turned on" by prolonged exposure to heat. That is, no new "complex specified information" was produced as the result of selection.

Well, that kind of response would be essentially irrelevant to the evolutionary implications of this experiment. What Nijhout and Co. have shown is that dramatic phenotypic changes can be induced as the result of selection for only a few generations, and that this can be correlated with the underlying hormonal physiology.

And besides, at least its an experiment, using real organisms and involving at least quasi-natural conditions. That is, it's light-years beyond the kind of intellectual masturbation typically performed by the average "intelligent design theorist," who declines to stoop to trivialities like empirical verification or publication in peer-reviewed mainstream scientific journals.

Complete evolutionist here. No doubts. And I started wondering a while ago, "haven't they demonstrated complex/macro speciation in the laboratory?" Seems they haven't. I'm kind of astounded. How do evolutionary biologists explain this? Email much appreciated if there's a reply, as I just googled across this blog... izforever128@gmail.com. Thanks, all.